The present invention relates to a sheet feed apparatus for an electrostatic copying machine or the like.
In a known electrostatic copying machine a clamp is spanningly attached to a pair of endless chains. The clamp grips the leading edge of a copy sheet for conveyance of the same into engagement with a photoconductive drum for transfer of a toner image from the drum to the sheet. Subsequently, the chains convey the sheet through a fixing unit which thermally fixes the toner image to the sheet to provide a permanent reproduction of an original document corresponding to the toner image.
The top sheet of a stack is fed into the bite of the clamp by a primary feed roller which is typically driven for one rotation. The clamp is spring loaded so as to be normally closed and is held open by a stationary release member when in a standby position relative to the stack of sheets. In the standby position, the chains and clamp are stationary in preparation for receiving the sheet. After the sheet reaches the clamp, the chain is driven and the clamp disengages from the release member, thereby closing and gripping the sheet for conveyance by the chains.
In practice, the clamp is insufficient to reliably grip the sheet in opposition to attractive electrostatic forces between the sheet and photoconductive drum during toner image transfer. For this reason, a pair of secondary feed rollers are provided between the primary feed roller and the chains. The secondary feed rollers are disengaged by a cam mechanism driven in interconnection with the chains so that the sheet may pass therebetween from the stack to the clamp. As the chains are driven to convey the sheet, the cam mechanism engages the secondary feed rollers to grip the sheet and feed the same, thereby assisting the clamp and chains.
However, such an apparatus is subject to frequent malfunction unless various adjustments are made with precision. The release member must be adjusted so as to open the clamp by the precise required amount. The gap between the secondary feed rollers in the disengaged state must be small enough to reliably guide the sheet therebetween but large enough not to frictionally resist movement of the sheet.
The clamp often fails to grip the sheet since it progressively closes as movement of the chains is initiated and the clamp disengages from the release member. The clamp and secondary feed rollers must grip the sheet at precisely the same time. If the clamp closes before or after the secondary feed rollers grip and begin to feed the sheet, the clamp has a good possibility of missing the leading edge of the sheet. The feed rollers will feed the sheet into engagement with the drum in such a manner that the sheet may become wrapped around the drum and cause an overall jam of the copying machine. If the sheet is fed into the fixing unit and becomes jammed therein, a similar overall jam of the copying machine will result and the sheet may catch on fire due to overexposure to heat in the fixing unit. In summary, it will be seen that unless a prior art sheet feed apparatus of the type described above is subjected to frequent and precise adjustment, it will frequently malfunction and may even constitute a first hazzard.